It is well known to provide a slat assembly comprising a slat connected to a plurality of tracks mounted on a fixed wing structure. The tracks are moveable relative to the fixed wing between an extended position in which the slat is deployed, and a stowed position, in which the slat is stowed on the fixed wing leading edge. The assembly typically comprises actuators for moving the tracks (for example by way of a rack and pinion connection). The shape of the tracks defines the position of the slat as it is deployed; the tracks may be a number of different shapes but are typically arcuate to define an arced forward and downward movement as the slat is deployed.
The tracks are connected to the slat, by a joint (sometimes referred to as a knuckle joint). In a known design (shown in FIGS. 1 and 2 and discussed in more detail in due course), a first track is connected to the slat by a master joint and a second, parallel, track is connected to the slat by a sub-master joint. Both the master and sub-master joints comprise links connecting the rear of the slat to their associated track. The links are mounted on spherical bearings such that the slat assembly is able to accommodate small changes to the slat geometry or position (for example bending or twisting) caused by, for example, aerodynamic loading.
The actuators used to move the slat are extremely robust and have met all the airworthiness requirements to date, but new airworthiness requirements may now require failure of an actuator to be considered. In the event of this theoretical failure of an actuator, only one of the tracks might be driveable between an extended and stowed position. In the above-mentioned slat assembly, the spherical bearings can accommodate a very small scale rotation of the slat, but under any substantive differential movement of the tracks (which would otherwise cause skew of the slat), the links would begin to interfere with the lugs on the slat/track and excessive loads may be generated. This would prevent the working track from continuing to operate without risking damage or failure of the joint.
Detection systems, to detect the misalignment of slats and to shut down the slat actuators in the event of misalignment, have been suggested. For example, EP0726201 describes an arrangement which detects when the slat becomes skewed beyond normal limits established for normal structural and thermal misalignments, and which stops further movement of the slats. By way of another example, EP 1088753 describes a skew detection system in which a control computer is arranged to shut down operation of the slat if the slat becomes skewed by more than a predetermined value. These detection systems can be complex. The detection systems also introduce a potential risk because in the event that the detection system malfunctions, the pilot may be able to continue to operate the slat assembly and thereby cause damage to the assembly.